One of the deepest images of the sky taken to date with NASA's Hubble Space Telescope reveals a population of faint blue galaxies which turn out to be the most common class of objects in the universe. Their distances are estimated to range from three to eight billion light-years, meaning that they were abundant when the universe was a fraction of its present age, but are rare or harder to find today because they have faded or self-destructed.

This picture, in combination with a series of images from the Hubble Space Telescope Medium Deep Survey that covers a larger area of sky, is allowing astronomers to solve the longstanding "faint blue galaxy mystery" by showing the true nature of these dim and remote objects. Deciphering the formation and evolution of these blue dwarf galaxies may provide new clues to understanding the process of galaxy evolution, including the formation of our Milky Way Galaxy.

Hubble's high resolution shows that most of these faint galaxies do not resemble elliptical and well-defined spiral galaxies that are common in the present universe. Instead, they have a wide variety of shapes suggesting that galaxy collisions and other interactions were more common in the past. The galaxies are blue because they are undergoing episodes of intense star-formation which produce a lot of young, hot, and blue stars.

This picture is a true-color image made from separate exposures taken in blue, green, and far-red light with the Wide Field and Planetary Camera 2. It required a total of 48 orbits around the Earth (amounting to roughly one day of exposure time) to make the observation and detect objects about four billion times fainter than the unaided eye can see (30th magnitude). The image resolution is about 0.06 arc seconds. The image covers a relatively small area of sky - only one tenth the diameter of the full moon - in the constellation Hercules.